Demodulating system for auxiliary channel in straight-through radio frequency repeater

ABSTRACT

The present invention discloses an FM microwave radio frequency repeater using the straight-through system for relaying signals of a plurality of channels having different carrier frequencies, each of the channels including an auxiliary channel. The system according to the invention is characterized in that the auxiliary channel has the signals thereof demodulated through frequency discrimination of a difference frequency signal obtained by mixing together two received microwaves.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for demodulating the signalsof auxiliary channels and an FM microwave radio frequency repeater usingthe straight-through system, with a plurality of channels havingdifferent carrier frequencies, each said channel including an auxiliarychannel.

2. Description of the Prior Art

With the progress in microwave semiconductor technology, a type ofmultichannel microwave radio frequency repeater having a simpleconstruction has come into practical use. This repeater is of the typethat comprises receiving frequency-modulated radio-frequency signals ofthe microwave frequency band and transmitting the signals thus receivedafter direct amplification thereof without converting them intointermediate frequencies, or transmitting said received signals afterslightly shifting the frequencies thereof for prevention of anyinterference due to the coupling of the antennas.

In the above-mentioned type of microwave radio frequency repeater,almost all of its transmission characteristics depend upon thecharacteristics of the amplifier provided therein. At the present timegood characteristics are available with such type of microwave radiofrequency repeater by virtue of the microwave semiconductors usedtherein. However, a repeater station equipped with such type ofmicrowave repeater still has a problem with respect to the demodulationof signals for the auxiliary channels. Usually the repeater station isnot attended by any operator but is periodically visited by maintenancepersonnel for inspection. This situation necessitates provision of anauxiliary channel in the microwave repeater, for emergencies, such astrouble with or an accident at the repeater station, for giving an alarmto the terminal station, or allowing various controls from the terminalstation to the unattended repeater station or communication from themaintenance personnel to the terminal station for necessaryarrangements, etc. Such auxiliary channel can find its effective use ina conventional type, i.e., non-straight-through type, of repeaterstation since, in such station, received signals can be demodulated by ademodulating circuit after their conversion into intermediatefrequencies. However, in the case of a microwave radio frequencyrepeater using the straight-through system, the repeater is no longerone of a simple construction if equipped with an independent microwavelocal oscillator for conversion of the picked-up signals intointermediate frequencies. Further, no satisfactory high-sensitivemicrowave frequency discriminator is currently available, although sucha discriminator is necessary for demodulating an auxiliary channelsignal through direct FM detection of the microwave. Thus, with today'sart the auxiliary channel is difficult to employ in a repeater stationequipped with such a microwave repeater using the straight-throughsystem.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a demodulating method forauxiliary channels and an FM microwave radio frequency repeater usingthe straight-through system, which is capable of easily obtainingdemodulated signals for the auxiliary channels without losing thefeature of simple construction possessed by such a straight-throughmicrowave radio frequency repeater.

It is a further object of the invention to provide a demodulating methodfor auxiliary channels and an FM microwave radio frequency repeaterusing the straight-through system adapted to relay microwaves in both anupline (go) and a downline (return) direction, said method and repeaterbeing capable of easily obtaining demodulated signals for the auxiliarychannels.

It is another object of the invention to provide a demodulating methodfor auxiliary channels and an FM microwave radio frequency repeaterusing the straight-through system adapted to relay signals of aplurality of channels having different carrier frequencies includingauxiliary channels, said method and repeater being capable of easilyobtaining demodulated signals for the auxiliary channels.

According to the invention there is provided a demodulating method forauxiliary channels and an FM microwave radio frequency repeater usingthe straight-through system, characterized in that microwaves of aplurality of microwave channels having different carrier frequencies,each including auxiliary channels, are received, two of the microwavesthus received are mixed to obtain a signal having a frequency of thedifference between said two branched frequencies, and said signal havingthe difference frequency is demodulated to obtain a demodulated signalfor said auxiliary channel.

Further features and advantages of the present invention will beapparent from the ensuing description with reference to the accompanyingdrawings to which, however, the scope of the invention is in no waylimited.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram illustrating the structure of a conventionalmicrowave radio frequency repeater using the straight-through systemprovided in repeater station;

FIG. 2 illustrates a frequency arrangement of the baseband signal andthe carrier wave used in the repeater shown in FIG. 1;

FIG. 3 is a block diagram illustrating the structure of a firstembodiment of the present invention;

FIG. 4 is a frequency arrangement of a baseband signal and the carrierwave used in the demodulating device of FIG. 3;

FIG. 5 is a block diagram illustrating the structure of a secondembodiment of the present invention;

FIG. 6 is a block diagram illustrating the structure of a thirdembodiment of the present invention;

FIG. 7 is a block diagram illustrating the structure of a fourthembodiment of the present invention;

FIG. 8 is a block diagram illustrating the structure of a fifthembodiment of the present invention, and;

FIG. 9 is one example of the circuit diagram of the mixer and thedemodulator used in the device according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is illustrated a block diagram for thestructure of a conventional repeater in which a frequency-modulatedmicrowave having a frequency F₁ (transmitted, e.g. in an uplinedirection) is received by an antenna 1a, then directed by circulator 2aand amplified by an amplifier 3a. The amplifier 3a, as well as anotheramplifier 3b, may be provided with a filter means for eliminatingspurious radiation and/or with an AGC circuit and a limiting circuit, ifrequired. Then, the signal thus amplified is applied to anothercirculator 2b to be transmitted through an antenna 1b. A microwavehaving a frequency F₂ (transmitted, e.g. in a downline direction) isreceived by the antenna 1b, subjected to the circulator 2b, amplified bythe amplifier 3b and transmitted by the antenna 1a after passing throughthe circulator 2a. This conventional type of repeater has almost all ofits transmission characteristics dependent upon the characteristics ofthe amplifiers 3a and 3b, which nowadays are good owing to the use ofmicrowave semiconductors as aforementioned. However, as notedhereinbefore, the repeater station in which the repeater is installed isunattended and is periodically visited by maintenance personnel forinspection. Therefore, an auxiliary channel has to be provided in therepeater for use in an emergency, such as trouble with or an accident atthe unattended repeater station, to give an alarm to the terminalstation, to allow various controls of the repeater station by theterminal station, or to permit the maintenance personnel to communicatewith the terminal station for necessary arrangements, etc.

Referring now to FIG. 2 illustrating the frequency arrangement, FIG.2(a) illustrates a frequency arrangement of the baseband signaltransmitted in the downline direction. It should be noted that 960channels of main communication channel signals (designated at C) arearranged in the range of from 60 KHz to 4 MHz, and two channels ofsignals respectively of from 0.3 to 4 KHz and from 4 to 8 KHz(designated at A and B, respectively) are provided in the lower part ofthe baseband for the auxiliary channel. Illustrated in FIG. 2(b) arebaseband signals transmitted in the upline direction. More specifically,two channels of signals, of respectively from 0.3 to 4 KHz and from 8 to12 KHz (designated at D and E, respectively), are provided for theauxiliary channel, and main communication channel signals are providedin the range of from 60 KHz to 4 MHz (designated at F). These basebandsignals in the upline and the downline directions are subjected tofrequency modulation by means of microwave carriers of the 6 GHZ band(Frequencies F₁ and F₂), separated from each other by 340 MHz, for useas transmitted-received waves for communication between terminalstations via repeater stations. Usually, terminal stations convert thereceived waves into intermediate frequencies and employ demodulatingcircuits, so that the auxiliary channel as aforementioned can beeffectively utilized. However, in a repeater station employing astraight-through microwave radio frequency repeater, provision of anindependent microwave local oscillator in said repeater for conversionof received waves into intermediate frequencies would deprive therepeater of its feature of simplicity in construction. In addition, nosatisfactory high-sensitive microwave frequency discriminator has yetbeen proposed, and such a discriminator would be required fordemodulation of auxiliary channel signals through direct FM detection ofmicrowaves.

Referring to FIG. 3, there is illustrated a block diagram illustrating afirst embodiment of the present invention, in which: reference numerals1a, 1b through 3a, 3b represent similar parts to those in FIG. 1; 4a, 4bdenote branching directional couplers for extracting part of the signalstransmitted through the up and down circuits; 5 represents a mixer forobtaining a difference frequency F₁ -F₂ between the signals; 6designates a demodulator, and; 7 denotes an output terminal fordemodulated signals for the auxiliary channel. The demodulator 6 isconstituted by a filter 6a for removing unnecessary waves other thancomponents F₁ -F₂, an amplifier 6b, a frequency discriminator 6c, and afilter 6d for picking out signals for the auxiliary channel band.

Signals having frequencies F₁ and F₂, extracted by the directionalcouplers 4a, 4b, are mixed together by the mixer 5 to produce afrequency of the sum of F₁ and F₂ and a frequency of the differencebetween F₁ and F₂, respectively. Of these produced frequencies, thedifference frequency F₁ -F₂, which corresponds to an intermediatefrequency, is picked out by the filter 6a of the demodulator 6, and isthen demodulated by the frequency discriminator 6c which can be aconventional type. More specifically, in the frequency arrangement shownin FIG. 4, F₁ and F₂ are microwave carriers of the 6 GHz band(transmitted-received signals) illustrated in FIG. 2(c). If thedifference between F₁ and F₂ is 340 MHz, the signal F₁ -F₂ shown in FIG.4(a) is a modulated signal with the frequency of 340 MHz as its owncarrier. Therefore, the modulated signals can be demodulated by thedemodulator 6 into demodulated signals as illustrated in FIG. 4(b). Thesignals in the range of from 60 KHz to 4 MHz, shown in FIG. 4(b),correspond to the sum of signals C and F of FIGS. 2(a) and (b), and thesignals in the range of from 0.3 KHz to 12 KHz correspond to signalsA+D, B and E. Since these signals can be obtained separately throughfrequency separation filters, both of the signals in the upline anddownline can be demodulated together at one time. Further, the signalA+D is intended for use in an order wire channel of the so-calledomnibus system, which is used when communication need not be carried outbetween particular repeater stations or between a particular terminalstation and a particular repeater station. Also in this system, theorder wire signals in both the upline and the downline can bedemodulated together.

Still further, since an ordinary telecommunication system is providedwith a spare channel intended for temporary use in the event of theoccurrence of trouble with the system, even if a channel of either ofthe upline or the downline becomes out of order, the demodulationoperation is not interrupted immediately.

FIG. 5 is a block diagram illustrating a second embodiment of theinvention. In this embodiment, in order to avoid the occurrence ofinterference due to the coupling of the receiving antenna and thetransmitting antenna of the repeater station, the received frequency andthe transmitted frequency, both transmitted in the same direction, areshifted so as to have a given difference, e.g., 340 MHz as in thepreceding embodiment (two-frequency system). Reference numerals 1a, 1bthrough 7 denote similar elements or parts to those in FIG. 3. Referencenumerals 8a, 8b are frequency shifting oscillators, and 9a, 9b frequencyshifting mixers. These mixers may preferably be provided with a bandpass filter function for removal of image frequencies. The wave of thefrequency F₁ received by antenna 1a has its frequency shifted to thefrequency F₂ by means of the frequency shifting mixer 9a, and saidreceived wave with the shifted frequency F₂ is shifted to the mixer 5after being extracted by the branching directional coupler 4a. The waveof the frequency F₁ received by antenna 1b is amplified by the amplifier3b, extracted by the directional coupler 4b, and then applied to themixer 5. Thus, a signal F₁ -F₂ is available at the output terminal ofthe mixer 5, so that the signal F₁ -F₂ can be demodulated by thedemodulator 6 as in FIG. 2.

FIG. 6 is a block diagram illustrating a third embodiment of theinvention. In this embodiment, all the received and transmitted wavestransmitted in the upline and the downline have different frequenciesfrom one another (four-frequency system). This embodiment can preventthe occurrence of interference due to the coupling of the antennas. InFIG. 6 identical reference numerals to those in FIG. 5 designateidentical parts or elements to those in FIG. 5. In FIG. 6, the wavereceived through the antenna 1a, has its frequency converted into thefrequency F₂ by means of the frequency shifting mixer 9a. The frequencyF₂ is then extracted by the branching directional coupler 4a and appliedto the mixer 5. The wave with the frequency F₂, received through theantenna 1b, has its frequency shifted to frequency F₄, then extracted bythe branching directional coupler 4b and, then, applied to the mixer 5.Thus, the mixer 5 produces an output signal having a frequency F₁ -F₂which can then be demodulated by the demodulator 6. Said branchingdirectional couplers 4 a, 4b may be located at the input sides of therespective frequency shifting mixers 9a, 9b, or at both of the input andoutput sides thereof. That is, branching and mixing may be performed atany place where those frequencies and signal strengths are obtainablewhich are in conditions wherein they are easy to handle as intermediatefrequencies by the demodulators and other devices. Since thecharacteristic features of the present invention resides in thedemodulation of the received waves, the above-description is directed toa case where modulated signals from the terminal stations aredemodulated. To effect modulation of signals of the auxiliary channel insuch microwave repeater using the straight-through system, the frequencyshifting oscillator with a frequency shifting mixer may be provided witha modulating function, if such frequency shifting oscillator is providedin said repeater, or a microwave phase modulator may be used, if suchfrequency mixer is not provided, to easily obtain modulated signals ineither case.

Referring now to a fourth embodiment of the present invention asillustrated in FIG. 7. Multiplexed receiving signals F₁, F₂, on tworoutes, which have reached a receiving antenna 11a are applied to acirculator 12a on the input end of the repeater. The circulator 12adelivers said received input signals F₁, F₂ to a band pass filter 13a.The band pass filter 13a allows only the signal F₁ to pass therethrough,while the input impedance of said filter takes a value of zero orinfinity with respect to the other signal with a different frequency.

The received signal F₁ thus passes through the band pass filter 13a,then is amplified up to a necessary level by an amplifier 14a, followedby being applied to a band pass filter 16a. The band pass filter 16aallows only the signal F₁ to pass therethrough, and the signal F₁, thushaving passed through the filter, is then impressed into a circulator12b on the output end. The circulator 12b transfers the input signal F₁to a transmitting antenna 11b. Thus, the received signal F₁ is amplifiedand again transmitted under the straight-through system.

On the other hand, the received signal F₂ is reflected at the inlet ofthe band pass filter 13a to be returned to the circulator 12a and passesthrough the same into another band pass filter 13b. The band pass filter13b allows only the signal F₂ to pass therethrough, so that the signalF₂ is applied to the amplifier 14b, which in turn amplifies the samesignal up to a necessary level and feeds the amplified signal to theband pass filter 16b.

The band pass filter 16b allows only the signal F₂ to pass therethrough.The signal F₂, having passed through said filter, is fed to the bandpass filter 16a after passing through the branching filter 12b on theoutput end. The output impedance of the band pass filter 16a takes avalue of zero or infinity with respect to frequencies other than that ofthe signal F₁. Accordingly, the signal F₂ is reflected by the band passfilter 16a to be returned to the branching circulator 12b. The signal F₂passes through the circulator 12b into the transmitting antenna 11b,through which the signal F₂ is radiated for transmision to the nextrepeater station or to the next terminal station. In this manner, thereceived signal F₂ is amplified and again transmitted under thestraight-through system. A part of each of the received signals F₁, F₂to be relayed is extracted from their respective transmission lines andapplied to the mixer 5 for mixing thereof, and the difference frequencyF₁ -F₂ is produced by the band pass filter of the demodulator 6 and,then, subjected to demodulation through the frequency discriminator.

Illustrated in FIG. 8 is a fifth embodiment according to the invention.In this embodiment, multiplexed signals F₁, F₂, transmitted on tworoutes and received by the receiving antenna 11a, are applied to a bandpass filter 17, which allows both of the signals F₁, F₂ to passtherethrough, and then, be amplified together by an amplifier 18 up to anecessary strength. They are further applied to a band pass filter 19,which permits both of them to pass therethrough, and then, they aretransmitted via a transmitting antenna 11b to the next repeater stationor to the next terminal station. Thus, the signals F₁, F₂ are relayed ina straight-through manner.

In such straight-through repeater, if the received signals F₁, F₂ beingrelayed are taken out of their common transmission line and applied to anon-linear element 20, such as a diode, a signal having a frequency ofthe sum of F₁ +F₂ and a signal having a frequency of the differencebetween F₁ and F₂ are produced. Out of these produced signals, thesignal having the difference frequency of F₁ -F₂ is extracted fordemodulation by means of the frequency discriminator in the demodulator6.

The signal having the difference frequency of F₁ -F₂ containsfrequency-modulated components possessed by the original signals F₁, F₂and said components are available at the output terminal of thedemodulator 6, so that they can be used as signals for the auxiliarychannels on the two routes in a similar manner to that of the embodimentof FIG. 7.

In the embodiments of FIG. 7 and FIG. 8, it should be noted that thereceived signals are directly amplified for the relaying thereof.However, the demodulating method for the auxiliary channel according tothe invention can, of course, include shifting the frequencies of thereceived signals into different frequencies for the relaying thereof.Also, in the case of a straight-through repeater having three or moreroutes, if the received signal on each route is subjected toamplification as in the embodiment of FIG. 7, first the sum of signalsF₁, F₂ on two of the routes, or the difference F₁ -F₂, is produced, andthen, the sum of said sum F₁ +F₂ and the signal F₃ on a third route orthe difference therebetween is produced, and then, demodulated. Thus,the signals F₁, F₂, F₃ can be utilized as signals for the auxiliarychannels of the three routes. A similar process to this can be appliedin the case of using a four or more route straight-through repeater.

FIG. 9 illustrates in detail the circuits of the mixer 5 and thedemodulator 6 used in the embodiments of the invention illustrated inFIG. 3, FIG. 5, FIG. 6 and FIG. 7. In FIG. 9, the directional couplers4a, 4b can usually be coaxial directional couplers, such as Model 3004manufactured by Narda Microwave Corporation. Isolators 21a, 21b areemployed in connection with the directional couplers 4a, 4b,respectively, for prevention of cross talk between the signalfrequencies F₁, F₂. A suitable isolator which can satisfy this purposeis, for instance, M3E-7200 manufactured by Microwave Associates, but theisolators 21a, 21b can be omitted if desired. The mixer 5 can be aDoubly Balanced Mixer incorporating four diodes as illustrated in FIG.9. A suitable model for the band pass filter 6a can be ModelTBP-340-50-3, of Telonic Industries Inc. A suitable model type for thewide band amplifier 6b can be Model G.P.D.-401 or G.P.D.-403, of AvantecInc. Model PLS-1 of Mini-Circuits Laboratory is suitable for use as thelimiter 22. The frequency discriminator 6c, as illustrated, isconstituted by a double-tuned discriminator incorporating transistorsQ₁, Q₂, Q₃ and diodes D₅, D₆. The band pass filter 6d is a low frequencyband pass filter constituted by inductance and capacitance elements, theoutput of which is amplified by a low frequency amplifier 23 andsupplied to the terminal 7.

As described in the foregoing, according to the method of the presentinvention, no microwave oscillator is necessary for demodulating signalsfor the auxiliary channel. Further, the demodulator does not require amicrowave frequency discriminator, but need only include just oneintermediate frequency discriminator, thus being very simple inconstruction.

What is claimed is:
 1. A demodulating method for auxiliary channel signals in a straight-through radio frequency repeater which relays microwave signals frequency-modulated by baseband signals composed of main communication channel signals and auxiliary channel signals having a different frequency band from that of said main communication channel signals, comprising:receiving a plurality of said microwave signals having different carrier frequencies; branching two of said plurality of microwave signals thus received; producing a difference frequency signal between said two branched microwave signals; and demodulating said difference frequency signal to regenerate said auxiliary channel signals.
 2. The method as claimed in claim 1, wherein said plurality of microwave signals having different carrier frequencies consist of both microwave signals transmitted in an upline and microwave signals transmitted in a downline.
 3. The method as claimed in claim 1, wherein said plurality of microwave signals having different carrier frequencies consist of microwave signals all transmitted in the same direction.
 4. A straight-through radio frequency repeater for relaying microwave signals, said microwave signals being frequency-modulated by baseband signals composed of main communication channel signals and auxiliary channel signals having a different frequency band from that of said main communication channel signals, comprising:a first receiving means for receiving upline microwave signals; a first microwave amplifier, operatively connected to said first receiving means, for amplifying said received upline microwave signals; a first branching directional coupler operatively connected to said first microwave amplifier for extracting part of said received upline microwave signals; a second receiving means for receiving downline microwave signals; a second microwave amplifier, operatively connected to said second receiving means, for amplifying said received downline microwave signals; a second branching directional coupler, operatively connected to said second microwave amplifier, for extracting part of said received downline microwave signals; a frequency mixer, operatively connected to said first and second branching couplers, for producing a difference frequency signal, said difference frequency signal being the difference between said received upline and downline microwave signals, and a demodulator, operatively connected to said frequency mixer, for demodulating said difference frequency signal, whereby said auxiliary channel signals are regenerated.
 5. The straight-through radio frequency repeater as claimed in claim 4, wherein said demodulator comprises:a first filter for removing unnecessary signals other than said difference frequency signal; an amplifier operatively connected to said first filter for amplifying said difference frequency signal; a frequency discriminator operatively connected to said amplifier for extracting said auxiliary channel signal from said difference frequency signal, and a second filter, operatively connected to said frequency discriminator, for passing signals in the frequency band of said auxiliary channel signals.
 6. The straight-through radio frequency repeater as claimed in claim 4, which further comprises:a first frequency shifting oscillator; a first frequency shifting mixer, operatively connected to said first microwave amplifier and said first frequency shifting oscillator, for mixing the outputs of said first frequency shifting oscillator and said first microwave amplifier to shift the frequency of said upline received microwave signal; a third microwave amplifier, operatively connected to said first frequency shifting mixer, for amplifying the output from said first frequency shifting mixer; a second frequency shifting oscillator; a second frequency shifting mixer, operatively connected to said second microwave amplifier and said second frequency shifting oscillator, for mixing the outputs of said second frequency shifting oscillator and said second microwave amplifier to shift the frequency of said downline received microwave signal, and a fourth microwave amplifier operatively connected to said second frequency shifting mixer for amplifying an output from said second frequency shifting mixer.
 7. The straight-through radio frequency repeater as claimed in claim 6, wherein said first frequency shifting mixer shifts the frequency of a received microwave signal having a frequency F₁ into a microwave signal having a frequency F₂, and said second frequency shifting mixer shifts the frequency of a received microwave signal having a frequency F₂ into a microwave signal having a frequency F₁.
 8. The straight-through radio frequency repeater as claimed in claim 6, wherein said first frequency shifting mixer shifts the frequency of a received microwave signal having a frequency F₁ into a microwave signal having a frequency F₂, and said second frequency shifting mixer shifts the frequency of a received microwave signal having a frequency F₃ into a microwave signal having a frequency F₄.
 9. The straight-through radio frequency repeater as claimed in claim 6, wherein modulation of said auxiliary channel signals is effected by modulating the outputs from said first and second frequency shifting oscillators.
 10. A straight-through radio frequency repeater for relaying multiplexed microwave signals, said multiplexed microwave signals being frequency-modulated by baseband signals composed of main communication channel signals and auxiliary channel signals having a different frequency band from that of said main communication channel signals, comprising:a receiving means for receiving said multiplexed microwave signals; a first microwave amplifier, operatively connected to said receiving means, for amplifying a first of said multiplexed microwave signals; a first branching directional coupler, operatively connected to said first microwave amplifier, for extracting part of the output from said first microwave amplifier; a second microwave amplifier, operatively connected to said receiving means, for amplifying a second of said multiplexed microwave signals; a second branching directional coupler operatively connected to said second microwave amplifier for extracting part of the output from said second microwave amplifier; a frequency mixer operatively connected to said first and second branching couplers for producing a difference frequency signal, said difference frequency signal being the difference between said first and second microwave signals, and a demodulator connected to said frequency mixer for demodulating said difference frequency signal whereby said auxiliary channel signals are regenerated.
 11. A straight-through radio frequency repeater for relaying multiplexed microwave signals, said multiplexed microwave signals being frequency-modulated by baseband signals composed of main communication channel signals and auxiliary channel signals having a different frequency band from that of said main communication channel signals, comprising:a receiving means for receiving said multiplexed microwave signals; a microwave amplifer, operatively connected to said receiving means, for amplifying a first and a second of said multiplexed microwave signals; a transmitting means, operatively connected to said microwave amplifier, for transmitting an output of said microwave amplifier; an extracting means, operatively connected to said transmitting means, for extracting part of said transmitted output of said microwave amplifier; a signal producing means, operatively connected to said extracting means, for producing a difference frequency signal said difference frequency signal being the difference between said first and second microwave signals; and a demodulator connected to said signal producing means for demodulating said difference frequency signal, whereby said auxiliary channel signals are regenerated.
 12. A straight-through radio frequency repeater as claimed in claim 11, wherein said signal producing means is a non-linear element.
 13. A straight-through radio frequency repeater as claimed in claim 12, wherein said non-linear element is a diode.
 14. A straight-through radio frequency repeater as claimed in claim 11, wherein said demodulator comprises:a first bandpass filter for filtering out frequencies other then said difference frequency signal; a wideband amplifier, operatively connected to said first bandpass filter, for amplifying said difference signal; a double tuned frequency discriminator, operatively connected to said wideband amplifier, for selecting signals in the frequency band of said auxiliary channel signals.
 15. A straight-through radio frequency repeater as claimed in claims 4, 6 or 10, wherein said demodulator comprises:a first bandpass filter for filtering out frequencies other than the frequency of said difference frequency signal; a wideband amplifier, operatively connected to said first bandpass filter, for amplifying said difference signal; a double tuned frequency discriminator, operatively connected to said wideband amplifier, for selecting signals in the frequency band of said auxiliary channel signals.
 16. A straight-through radio frequency repeater as claimed in claim 15, wherein each said branching directional coupler is a coaxial directional coupler. 